This invention relates generally to fuel burners, and more particularly to an apparatus which efficiently burns fuel to produce a stable, non-contaminating blue flame over a broad operating range, the fuel being a liquid hydrocarbon to which a gaseous hydrocarbon may be added.
Conventional forms of fuel burners, even when carefully adjusted, often operate inefficiently and produce yellow or orange flames characterized by incomplete combustion. This gives rise to soot deposits in the flue and to the discharge into the atmosphere of particulates. Hence such burners not only fail to operate economically, but they act to contaminate the atmosphere. The concern of the present invention is with burners which are adapted to effect full combustion of fuel supplied thereto and to produce a stable, non-contaminating blue flame.
Liquid fuel burners are known in which gasification of the liquid fuel is effected by recirculating a portion of the hot combustion gas into admixture with the fuel in order to promote full combustion. Typical of commercial blue-flame, liquid fuel burners which make use of a gasification system is the THERMAL HV (high velocity) burner manufactured by the Thermal Research and Engineering Corp. of Conshohocken, Pa., a division of Trane Thermal Company, under U.S. Pat. Nos. Re 24,682; 2,839,128 and 3,042,105, among others.
In a THERMAL HV burner, liquid fuel is injected into the inlet of a flow passage leading to a combustion zone, combustion air being fed into the same passage. The mixture of fuel and air is ignited in the combustion zone, and a portion of the resultant hot combustion products are drawn back into the inlet of the flow passage through a feedback passage. This recirculated hot gas serves to promote vaporization of the fuel-air mixture before it is ignited to ensure full combustion thereof.
In the THERMAL HV burner, the recirculation of the hot combustion gas is induced by forcing all of the incoming, relatively cool combustion air into the flow passage through a throat section of reduced diameter, thereby creating a low-pressure Venturi effect serving to suck the hot gas from the feedback passage into the inlet of the flow passage to prevaporize the liquid fuel. The pressure differential or Venturi effect required to draw in the hot combustion gas depends primarily on the mass velocity or momentum of the incoming combustion air passing through the throat section; the greater this momentum, the stronger the suction force for drawing in the hot gas.
It is well known that to achieve proper conversion efficiency, one must maintain close to a stoichiometric air/fuel ratio in the burning zone over the full range of operating conditions from high to low. Every chemical reaction has its characteristic properties. For example, when methane unites with oxygen in complete combustion, 16 grams of methane require 64 grams of oxygen. If, therefore, at a given operating setting of the burner, where a given amount of liquid fuel is being fed therein, a proper proportion of air must also be introduced to obtain a stoichiometric ratio producing full combustion and a blue flame.
If one thereafter turns down the burner to reduce the volume of liquid fuel being admitted therein, one must at the same time lower the flow rate of incoming air to maintain the proper ratio therebetween. As a consequence, in a blue flame burner of the THERMAL HV type and in other burners operating along similar principles, the pressure differential available to promote recirculation of hot gas falls off as the burner is turned down, the fall-off curve being steep when the burner has a wide operating range.
These known burner arrangements therefore provide proper hot gas recirculation and function at their optimum efficiency only when the burner is turned almost all the way up; for as one turns down the burner, there is a concomitant loss of suction force and weakened recirculation of the hot gases necessary to effect pre-vaporization of the liquid fuel. The liquid fuel, instead of being pre-vaporized before entry into the combustion chamber, enters therein in the form of atomized droplets. As a result, combustion is incomplete and objectionable contaminants are generated.
An important factor which comes into play in determining the conversion efficiency of a liquid fuel burner is atomization of the fuel; for the finer the atomization, the more effective is the conversion process. Generally, atomization is carried out by steam or pressurized air. As noted in the Combustion Handbook, published by the North American Manufacturing Co. of Cleveland, Ohio, it is customary to classify atomizing streams as high pressure streams (above 5 psig) and as low pressure streams, typically in the order of 2 psig.
A further classification is based on the nature of the mixing process; that is, whether the fuel and atomizing streams are internally or externally mixed. Internal mixing usually involves high pressure streams, while external mixing is of the low pressure variety.
In an internal mixing fuel atomizer, the atomizing air stream and the liquid fuel are introduced into a mixing chamber where vigorous agitation takes place at relatively high velocities to create a finely atomized mixture. In an external mixing system, the liquid fuel to be atomized is discharged from a nozzle and is then subjected to the atomizing stream.
In an atomizer, it is generally desirable that the mass flow ratio of atomizing air-to-liquid fuel (i.e., the nozzle air/fuel ratio) be minimized. Moreover, since a source of compressed air or other atomizing medium is usually included in a packaged atomizer system, in order to eliminate the need for large atomizing power drive trains, the pressure requirements of the atomizing stream should be kept to a minimal level.
The problem heretofore encountered in producing a fuel-atomizing system is that while one can optimize the nozzle air/fuel ratio and minimize the power requirements under fixed operating conditions, it is difficult to attain satisfactory atomization over wide liquid flow turn-down ranges with a minimum of absorbed power at a low air-to-liquid mass flow rate at the maximum capacity.